diff --git a/packages/assets-controller/CHANGELOG.md b/packages/assets-controller/CHANGELOG.md
index ad35f5f1fd..a0eba5b54d 100644
--- a/packages/assets-controller/CHANGELOG.md
+++ b/packages/assets-controller/CHANGELOG.md
@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - `decimals: 0` is treated as valid; `name` and `symbol` are not required.
   - Decimals resolution in `#handleBalanceUpdate` and the manual-fetch path no longer relies on `??` to fall through from state to pipeline metadata. A new `#pickValidDecimals` helper picks the first source whose `decimals` is finite and non-negative, so a stale `decimals: NaN` (or `decimals: -1`) in state can no longer shadow the chain-status stub's `decimals: 18` and silently produce `amount: '0'` while `assetsInfo` reports `decimals: 18`.
   - `#convertToHumanReadable` now defensively returns `'0'` when `decimals` isn't a finite non-negative number or when the raw balance can't be parsed, matching the existing safe fallback used in the error path.
+- The mUSD (`MetaMask USD`) contract address stored in `defaults.ts` is now EIP-55 checksummed (`0xacA92E438df0B2401fF60dA7E4337B687a2435DA`) ([#8786](https://github.com/MetaMask/core/pull/8786)). Previously the address was all-lowercase, causing the CAIP-19 keys pre-seeded into `assetsInfo` by `buildDefaultAssetsInfo()` to differ from the checksummed keys written by data sources (which always normalise asset IDs via `normalizeAssetId`). The mismatch resulted in two separate `assetsInfo` entries for the same token after the first balance or token-data poll.
 
 ## [7.1.1]
 
diff --git a/packages/assets-controller/src/AssetsController.test.ts b/packages/assets-controller/src/AssetsController.test.ts
index da37526a53..ab609ea176 100644
--- a/packages/assets-controller/src/AssetsController.test.ts
+++ b/packages/assets-controller/src/AssetsController.test.ts
@@ -260,6 +260,23 @@ describe('AssetsController', () => {
         selectedCurrency: 'usd',
       });
     });
+
+    it('pre-seeds assetsInfo with EIP-55 checksummed CAIP-19 keys', () => {
+      // Regression: MUSD_ADDRESS was previously all-lowercase, so
+      // buildDefaultAssetsInfo() produced lowercase CAIP-19 keys while data
+      // sources (which call normalizeAssetId) wrote checksummed keys.
+      // After the first balance poll both keys existed in assetsInfo.
+      const defaultState = getDefaultAssetsControllerState();
+      const assetIds = Object.keys(defaultState.assetsInfo);
+      expect(assetIds.length).toBeGreaterThan(0);
+      // Every erc20 asset ID must contain at least one uppercase hex letter
+      // (EIP-55 checksum property) so that keys match normalizeAssetId output.
+      const erc20Ids = assetIds.filter((id) => id.includes('/erc20:'));
+      expect(erc20Ids.length).toBeGreaterThan(0);
+      for (const id of erc20Ids) {
+        expect(id).toMatch(/\/erc20:0x[0-9a-fA-F]*[A-F][0-9a-fA-F]*/u);
+      }
+    });
   });
 
   describe('constructor', () => {
diff --git a/packages/assets-controller/src/defaults.ts b/packages/assets-controller/src/defaults.ts
index 963008a604..edb5be687b 100644
--- a/packages/assets-controller/src/defaults.ts
+++ b/packages/assets-controller/src/defaults.ts
@@ -6,10 +6,18 @@ import type {
 } from './types';
 
 /**
- * Address of MetaMask USD (mUSD) — same canonical contract address
- * across every chain we deploy it to.
+ * EIP-55 checksummed address of MetaMask USD (mUSD) — same canonical contract
+ * address across every chain we deploy it to.
+ *
+ * Must be checksummed so that the CAIP-19 asset IDs produced by
+ * `musdAssetId()` (and therefore the keys seeded into `assetsInfo` by
+ * `buildDefaultAssetsInfo()`) match the keys written by data sources, which
+ * always pass IDs through `normalizeAssetId` → `toChecksumAddress` before
+ * emitting a `DataResponse`. Using a lowercase address would cause the
+ * pre-seeded keys to diverge from the data-source keys, leaving a duplicate
+ * entry in `assetsInfo` after the first balance or token-data poll.
  */
-const MUSD_ADDRESS = '0xaca92e438df0b2401ff60da7e4337b687a2435da';
+const MUSD_ADDRESS = '0xacA92E438df0B2401fF60dA7E4337B687a2435DA';
 
 /**
  * Hardcoded metadata for MetaMask USD. Pre-seeding this in default
@@ -63,14 +71,14 @@ export const CHAINS_WITH_DEFAULT_TRACKED_ASSETS: ReadonlySet<ChainId> = new Set(
 
 /**
  * Pre-seeded metadata for every default tracked asset, keyed by the
- * lowercase CAIP-19 id so callers can look up without worrying about
- * checksum case.
+ * checksummed CAIP-19 id. All callers must pass a checksummed asset ID;
+ * use `normalizeAssetId` to ensure the correct format before looking up.
  */
 export const DEFAULT_ASSET_METADATA: ReadonlyMap<string, AssetMetadata> =
   new Map<string, AssetMetadata>([
-    [musdAssetId('eip155:1' as ChainId).toLowerCase(), MUSD_METADATA],
-    [musdAssetId('eip155:59144' as ChainId).toLowerCase(), MUSD_METADATA],
-    [musdAssetId('eip155:143' as ChainId).toLowerCase(), MUSD_METADATA],
+    [musdAssetId('eip155:1' as ChainId), MUSD_METADATA],
+    [musdAssetId('eip155:59144' as ChainId), MUSD_METADATA],
+    [musdAssetId('eip155:143' as ChainId), MUSD_METADATA],
   ]);
 
 /**
@@ -89,14 +97,14 @@ export function getDefaultTrackedAssetsForChain(
 /**
  * Look up pre-seeded metadata for a default tracked asset.
  *
- * @param assetId - CAIP-19 asset id (any case).
+ * @param assetId - CAIP-19 asset id (must be EIP-55 checksummed for EVM tokens).
  * @returns The metadata if the asset is a default tracked asset,
  * otherwise `undefined`.
  */
 export function getDefaultAssetMetadata(
   assetId: Caip19AssetId,
 ): AssetMetadata | undefined {
-  return DEFAULT_ASSET_METADATA.get(assetId.toLowerCase());
+  return DEFAULT_ASSET_METADATA.get(assetId);
 }
 
 /**